Micromechanical thermal-conductivity sensor having a porous cover

ABSTRACT

A micromechanical thermal-conductivity sensor is provided which includes a thermally insulated diaphragm formed by a recess in a base plate exhibiting poor thermal conductivity. At least one heating element is applied on the diaphragm, at least one temperature-dependent electrical resistor is applied on the diaphragm for measuring the temperature of the diaphragm, as well as at least one further temperature-dependent electrical resistor is applied outside of the diaphragm on the base plate for measuring the ambient temperature. On one or both of its sides, the diaphragm is covered by a porous cover plate permitting gas exchange by diffusion, a cavity being left open between the diaphragm and the porous cover plate.

FIELD OF THE INVENTION

The present invention relates to a micromechanical thermal-conductivitysensor. In particular, it relates to a micromechanicalthermal-conductivity sensor which includes a thermally insulateddiaphragm formed by a recess in a base plate exhibiting poor thermalconductivity, at least one heating element applied on the diaphragm, atleast one temperature-dependent electrical resistor applied on thediaphragm for measuring the diaphragm temperature (T_(M)), as well as atleast one further temperature-dependent electrical resistor, appliedoutside the diaphragm on the base plate, for measuring the ambienttemperature (T_(U)). The present invention also relates to a method forproducing the thermal-conductivity sensor, as well as its use.

BACKGROUND INFORMATION

The measurement of thermal conductivity is used frequently for analyzinggases, particularly for the quantitative analysis of two-component gasmixtures.

In principle, it holds true that the thermal conductivity of a gas orgas mixture varies with the mass of the gas molecules, theirconcentration and the temperature. If the (mixed-) thermal conductivityof the gas or the gas mixture is measured at a known gas temperature andfor known components, then it is possible to exactly determine theconcentrations of individual components of the gas mixture based on thethermal conductivity. Above all, the concentrations of hydrogen (H₂) andhelium (He) in the mixture with other gases such as air, oxygen (O₂),nitrogen (N₂), ammonia (NH₃), argon (Ar), carbon dioxide (CO₂), carbonmonoxide (CO), chlorine (Cl₂), hydrogen sulphide (H₂S), methane (CH₃),nitrogen monoxide (NO), dinitrogen monoxide (N₂O) and water vapor (H₂O)may be measured well, because these gases have particularly high thermalconductivity compared to the other gases. Thus, the thermal conductivityof hydrogen (H₂) is λ_(hydrogen)=0.84 Wm⁻¹K⁻¹ and the thermalconductivity of helium (He) is λ_(helium)=0.6 Wm⁻¹K⁻¹, while the thermalconductivity of air λ_(air)=0.012 Wm⁻¹K⁻¹ is less by approximately afactor 5-7.

To measure the thermal conductivity, a body is brought to a temperatureT_(K) which is higher than the ambient temperature T_(U). A gas(mixture) surrounding the body is generally at ambient temperature. Forexample, if the temperature difference ΔT=T_(K)−T_(U) is held constant,then the heating power P_(H) necessary for this purpose is a measure forthe thermal conductivity of the surrounding gas or gas mixture. Heatingpower P_(H) is directly proportional to constantly retained temperaturedifference ΔT; the proportionality constant results from the product ofthe thermal conductivity λ and a (constant) geometry factor K which is afunction of the measuring device. This correlation is described inequation (1):P _(H) =KλΔT  (1)where P_(H) designates the heating power, ΔT designates the temperaturedifference, K designates the geometry factor and λ designates thethermal conductivity.

Recently, micromechanical thermal-conductivity sensors based on siliconare increasingly being developed to determine thermal conductivity. Incontrast to conventional thermal-conductivity sensors, these sensorshave the feature of low power consumption, which essentially is nogreater than the power consumption of the electronic equipment neededfor the signal processing. Moreover, the miniaturized sensors possessshort response times (time constants) which generally can only beachieved by the acceptance of a forced traversal with the measuring gas,i.e., a flow dependency, in the case of conventional sensors. In thelast analysis, however, this makes universal use impossible. Finally,such micromechanical thermal-conductivity sensors based on silicon areeconomical to produce, because it is possible to fall back on customarymethods for the production of integrated semiconductor components.

The design of a typical conventional micromechanicalthermal-conductivity sensor based on silicon is shown in FIG. 1 and isdescribed below.

A fundamental problem when measuring the thermal conductivity of asurrounding gas is the heat transfer caused by convection of the gas,accompanied by a falsification of the actual thermal-conductivity value.A convective heat transfer may result from external gas movements whichbecome noticeable in the sensor, but also from the temperaturedifference, necessary for the measurement, between the heating elementand diaphragm, respectively, and the gas.

It may be that a forced traversal of the thermal-conductivity sensorwith the gas to be measured is definitely desired in order to permit arapid gas exchange and therefore short response times; however, formicromechanical thermal-conductivity sensors having very small measuringvolumes and an inherently rapid gas exchange thereby permitted, anyconvective heat transfer is regarded as disadvantageous.

To prevent convective heat transfer, the sensor is usually used in sucha way that the gas volume near the diaphragm remains quiet. This may beachieved, for example, by covering the diaphragm with a cover plate.

For example, German Patent No. DE 37 252 describes a micromechanicalthermal-conductivity sensor for measuring the thermal conductivity of agas mixture, in which an insulator layer is applied on a support platemade of silicon, meander-shaped thin-film resistors being applied on theinsulator layer by vapor deposition or sputtering. In the region of thethin-film resistors, the insulator layer is undercut so that in thesupport plate, a hollow is obtained which forms the lower part of thesensor measuring chamber. Resting on the support plate having thethin-film resistors is a silicon cover plate into which a hollow isetched at the height of the thin-film resistors which forms the upperpart of the measuring chamber. The cover plate has an opening which, asa diffusion channel, makes it possible for the gas mixture to enter themeasuring chamber. The exchange of gas in the lower hollow of themeasuring chamber takes place through cutouts in the insulator layer.

The disadvantage in this thermal-conductivity sensor is that thedimensions of the diffusion channel must in any case be selected sothat, first of all, the gas in the measuring chamber is exchanged asquickly as possible by diffusion through a large opening, but secondly,gas movements which occur outside of the measuring chamber are nottransferred into the measuring chamber, which requires a small opening.However, this objective can only be achieved as a compromise between thetwo requirements, a given design of the diffusion channel generally nolonger permitting a universal application.

SUMMARY

An object of the present invention is to overcome the disadvantages ofthe conventional micromechanical thermal-conductivity. In particular, auniversally applicable thermal-conductivity sensor is provided in whicha convective heat transfer through the gas or gas mixture to be measuredis avoided to the greatest extent possible.

According to the present invention, a micromechanicalthermal-conductivity sensor is described which includes a thermallyinsulated diaphragm formed by a recess in a base plate exhibiting poorthermal conductivity, at least one heating element (resistance element)applied on the diaphragm, at least one temperature-dependent electricalresistor applied on the diaphragm for measuring the temperature of thediaphragm, as well as at least one further temperature-dependentelectrical resistor applied outside of the diaphragm on the base platefor measuring the ambient temperature, and is characterized in that onone or both of its sides, the diaphragm is covered by a porous coverplate permitting gas exchange by diffusion, a cavity being left openbetween the diaphragm and the porous cover plate. Thus, thethermal-conductivity sensor of the present invention may be providedwith one or two porous cover plates, i.e., may have one or two cavitiesbetween the diaphragm and cover plate(s).

The porous cover plate(s) advantageously permit diffusion of thesurrounding gas or gas mixture into the cavity forming the measuringspace, without at the same time the measurement result being falsifiedby a convection current of the gas. Because of the small dimension ofthe cavity or cavities, particularly due to its/their low height, theconvective heat transfer in the direction of a cover plate caused by thetemperature difference between the diaphragm and heating element,respectively, and the cover plate is substantially reduced, so that aconvection current of the gas to be measured in a cavity is minimized onthe whole and is essentially prevented. The response time of the sensoris sharply reduced by the gas exchange rapidly taking place. Moreover,the sensitivity of the sensor may be increased, because the minimizedconvection current of the gas or gas mixture to be measured provides thepossibility of using a relatively great temperature difference betweenthe diaphragm and the measuring gas. There is also the possibility ofusing the sensor universally, regardless of the installation situation.

As described above, the porous cover plate may be applied on one or bothsides of the (flat) diaphragm. If the porous cover plate is applied onthat side of the base plate which has the recess for forming thediaphragm, after the cover plate is applied, this recess forms a cavitywhich is used as the measuring space for the sensor. If the porous coverplate is applied on the side of the base plate opposite this side, thenthe porous cover plate must have a recess situated opposite thediaphragm, this recess forming a cavity after the cover plate isapplied. This cavity then forms a measuring space for thethermal-conductivity sensor. The sensor may therefore optionally beprovided with one or two cavities (i.e., measuring spaces) for the gas(mixture) to be measured.

The base plate is preferably made of silicon. This has the advantagethat for the further processing, it is possible to resort to the methodsused in semiconductor technology such as vapor deposition methods,sputtering methods, photolithographic methods, etching methods andpassivation methods. It is thus possible to produce the sensors in acost-effective manner. In particular, a plurality of sensors accordingto the present invention may be produced from a single silicon wafer.

The porous cover plate is preferably made from a porous ceramicmaterial, particularly SiC and Al₂O₃. If applicable, the porous coverplate is made at least partially of porous silicon.

Preferably, the thermal expansion coefficient of the ceramic material isessentially equal to the thermal expansion coefficient of silicon, or atleast is close to it. In this case, the cover plate(s) and the baseplate and diaphragm, respectively, have an equal thermal expansion,which means thermally caused stresses after applying the cover plate onthe base plate may advantageously be avoided or minimized.

The thickness of the base plate used for producing the sensor ispreferably in the range of 200-600 μm, while the diaphragm preferablyhas a thickness in the range of 0.6-2 μm. Moreover, the diaphragm ischaracterized by an area preferably in the range of 0.25-4 mm².

If the pores of the porous cover-plate material are selected to be smallenough, the gas (mixture) passing through may be filtered in anextremely advantageous manner during the diffusion. Depending upon thepore size of the porous material, foreign matter such as soot and dirtparticles or microbiological impurities may be removed from the gas(mixture) by this filtering.

The heating element and the temperature-dependent electrical resistorsare preferably made of silver (Ag), gold (Au), nickel (Ni) or platinum(Pt). In one particularly advantageous refinement of the presentinvention, the cross-section of the heating element is of different sizein the region of the contacting and in the heating zone. This may beadvantageously used when sinter-fusing the cover plate onto the baseplate.

The temperature-dependent electrical resistors and the heating element,respectively, may be protected from the influence of chemicallyaggressive gases and gas mixtures by a passivation layer. A siliconcompound such as silicon oxide (SiO₂) and silicon nitride (Si₃N₄)primarily present themselves as substances for the passivation layer.

The thermal-conductivity sensor according to the present invention maybe produced by adhering the porous cover plate onto the base plate. Anadhesion of the base plate and cover plate presents itself in particularwhen the materials used have a different thermal expansion coefficient,since an adhesive connection is able to compensate for a differentthermal expansion. This prevents thermally caused stresses in the joinedmaterials. In particular, silver-filled adhesive agents having adaptedcoefficients of thermal expansion may be used as adhesive agents.

If there is only a slight difference in the thermal expansioncoefficients between the porous cover plate made, for instance, of aporous ceramic material, and the base plate, it is also possible todirectly encase the base plate with ceramic by sinter-fusing. For thatpurpose, prior to applying the material of the porous cover plate to besintered, a resist layer, to be removed after the sintering process, forforming the cavity in the porous cover plate is applied on the baseplate.

The heat necessary for the sintering process may be supplied fromoutside. However, the possibility also exists of effecting the sinteringprocess solely by the resistance heat of the heating element. In thiscase, the diameter of the heating element, e.g., a platinum wire, may besubstantially thicker in the heating zone than in the region of itscontacting. In a particularly advantageous manner, the resist layer mayalso be removed by the resistance heat of the heating element, and thusa cavity having defined dimensions may be formed.

Moreover, the thermal-conductivity sensor of the present invention maybe manufactured monolithically, avoiding the joining process between thebase plate and cover plate. To this end, a layer of porous silicon isproduced on the actual sensor base material of silicon; this is carriedout in such a way that a cavity is formed between the subsequentlyformed diaphragm and the layer of porous silicon.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, several exemplary embodiments of thethermal-conductivity sensor according to the present invention, as wellas methods for manufacturing it, are presented with reference to thefigures.

FIG. 1 shows the construction of a conventional micromechanicalthermal-conductivity sensor in plan view (lower part) and incross-section along the indicated line of intersection (upper part).

FIG. 2 shows a specific example embodiment of the thermal-conductivitysensor according to the present invention, in which a porous cover plateis stuck onto the silicon base plate on both sides of the diaphragm.

FIG. 3 shows another specific example embodiment of thethermal-conductivity sensor according to the present invention, in whicha porous cover plate is sinter-fused onto the silicon base plate on bothsides of the diaphragm.

FIG. 4 shows two further specific example embodiments of thethermal-conductivity sensor according to the present invention, in whicha porous cover plate of silicon is sinter-fused onto the silicon baseplate on both sides of the diaphragm.

DESCRIPTION OF EXAMPLE EMBODIMENTS

As the lower part of FIG. 1 shows, in a typical conventionalmicromechanical thermal-conductivity sensor, a heating element(resistance element) 3 in the form of a Pt-resistance wire and atemperature-dependent electrical resistor 5 for detecting diaphragmtemperature T_(M) are arranged in a meander form on silicon base plate 2and diaphragm 1, respectively. Located outside of diaphragm 1 is afurther temperature-dependent electrical resistor 7 for detectingambient temperature T_(U). Base plate 2, together with the appliedelectrical structures, forms a silicon sensor chip for measuring thethermal conductivity. In the upper part of FIG. 1, it can be seen thatdiaphragm 1 is formed as a recess in base plate 2.

To measure the thermal conductivity of the surrounding gas or gasmixture, for example, the heating power of the heating element needed tokeep the difference between T_(M) and T_(U) constant is determined.Because of the small mass of the thin diaphragm and the materialsapplied thereon, very small thermal time constants may be achieved whichare typically on the order of magnitude of milliseconds and below.Moreover, because of the thin diaphragm, the dissipation of heat via thesurrounding gas is substantially greater than via the diaphragm materialitself, resulting in great sensitivity of the sensor to changes in thethermal conductivity of the surrounding gas. The great sensitivity ofthe sensor also opens up the possibility of reducing the heatdissipation all in all, i.e., of lowering temperature difference ΔTbetween the diaphragm and the surroundings, which is reflected in areduction of the electrical power loss of the sensor.

FIG. 2 shows a specific example embodiment of the thermal-conductivitysensor according to the present invention. A cover plate 8 made ofporous ceramic is applied on both sides of the silicon sensor chip whichis made up of base plate 2 together with applied current-conductivestructures 3, 5. The recess of base plate 2, together with cover plate8, forms a cavity 6 which is used as the measuring space for the sensor.On the other side of diaphragm 1, a recess is formed in cover plate 8;after cover plate 8 is applied on base plate 2, the recess forms acavity 4 which is used as a further measuring space for the sensor. Bothcover plates 8 are pasted onto the silicon sensor chip. Adhesive layers9 are capable of compensating for any differences in the thermalexpansion of ceramic cover plates 8 and the silicon sensor chip.

FIG. 3 shows another specific example embodiment of thethermal-conductivity sensor according to the present invention. In thiscase, a cover plate made of porous ceramic material is sinter-fused ontothe silicon sensor chip on both sides of diaphragm 1. Because of thelack of possibility to compensate for different thermal expansionsbetween the cover plates and the silicon sensor chip, cover plates 8 areonly sinter-fused on if the thermal expansion coefficient of the ceramicmaterial of the cover plates and the thermal expansion coefficient ofthe silicon sensor chip are very similar or identical. Before applying apaste or dispersion of the ceramic material for cover plates 8 on thesilicon sensor chip, a resist layer 22 is applied in the region ofheating element 3 and/or diaphragm 1, which is removed after the ceramicmaterial is sintered. Cavities 4, 6 remain at the location of previousresist layer 22.

This production sequence is illustrated in FIG. 3. FIG. 3 a) showssilicon sensor chip 2. FIG. 3 b) shows silicon sensor chip 2 withapplied resist layer 22. FIG. 3 c) shows silicon sensor chip 2 withapplied resist layer 22, as well as the ceramic material of cover plates8 applied on both sides of the silicon sensor chip. Finally, FIG. 3 d)shows cavities 4, 6 formed after the sintering process and removal ofresist layer 22.

Both the sintering of the ceramic material of cover plates 8 and removalof resist layer 22 may be brought about by the resistance heat ofPt-resistance wire 3. For this purpose, the resist layer is made of aless heat-resistant, e.g., organic (sacrificial) material. A definedcavity may be created between the Pt-resistance wire and the porousceramic encasing by way of the layer thickness of this materialdecomposing during the sintering process. To this end, Pt-resistancewire 3 has a different cross-section for the contacting and in theregion of the heating zone. A bond 11 is applied on silicon sensor chip2 for the electrical contacting.

FIG. 4 shows two further specific example embodiments of thethermal-conductivity sensor according to the present invention. In thesespecific embodiments, instead of a porous ceramic, porous silicon isused for cover plates 8. Because the thermal expansion coefficients ofthe cover plates and the silicon sensor chip are the same, thermallycaused material stresses are avoided from the start.

FIG. 4 a) shows silicon sensor chip 2. FIG. 4 b) shows a silicon wafer12 having a porous region 13, silicon wafer 12 having a recess in porousregion 13. Silicon wafer 12 is placed on silicon sensor chip 2 on bothsides of diaphragm 1, in each case porous region 13 coming to rest overdiaphragm 1.

FIG. 4 c) shows a variant, in which silicon wafer 12 is positioned insuch a way on the side of diaphragm 1 on which the recess of siliconsensor chip 2 is located, that the cavities of silicon wafer 12 and ofsilicon sensor chip 2 form a common cavity having a larger volume.

FIG. 4 d) shows a further variant, in which silicon wafer 12 ispositioned in such a way on the side of diaphragm 1 on which the recessof silicon sensor chip 2 is located, that the cavity of silicon sensorchip 2 remains unchanged. The arrangement of silicon wafer 12 on therespective opposite diaphragm side is the same for both variants ofFIGS. 4 c) and 4 d).

In one particularly advantageous modification of the two variantsindicated above, the costly joining process can be avoided, in that alayer of porous silicon is already produced on sensor chip 2, a cavitybeing formed between diaphragm 1 and the porous silicon layer. In thisway, a monolithic thermal-conductivity sensor is obtained.

The micromechanical thermal-conductivity sensor according to the presentinvention is advantageously used for analyzing gases and gas mixtureswhich are analyzed in particular with respect to their type andconcentration. In this context, binary gas mixtures are primarily suitedfor a quantitative analysis. Because of their comparatively high thermalconductivity, hydrogen gas (H₂) and helium (He) may be analyzed quicklyand easily.

1. A micromechanical thermal-conductivity sensor, comprising: athermally insulated diaphragm formed by a recess in a base plateexhibiting poor thermal conductivity; at least one heating elementapplied on the diaphragm; at least one temperature-dependent electricalresistor applied on the diaphragm for measuring a temperature of thediaphragm; and at least one further temperature-dependent electricalresistor applied outside the diaphragm on the base plate for measuringan ambient temperature; wherein the diaphragm is covered on at least oneof its sides by a porous cover plate with a plurality of pores whichpermits gas exchange through diffusion, a cavity being left open betweenthe diaphragm and the porous cover plate, the porous cover plate beingsecured on the base plate, and being made of at least one of a porousceramic material or at least partially of porous silicon.
 2. Themicromechanical thermal-conductivity sensor as recited in claim 1,wherein the porous cover plate has a recess forming the cavity.
 3. Themicromechanical thermal-conductivity sensor as recited in claim 1,wherein the base plate is made of silicon.
 4. The micromechanicalthermal-conductivity sensor as recited in claim 1, wherein the porouscover plate is made of at least one of SiC and Al2O3.
 5. Themicromechanical thermal-conductivity sensor as recited in claim 4,wherein a thermal expansion coefficient of material of the porous coverplate is substantially the same as a thermal expansion coefficient ofsilicon.
 6. The micromechanical thermal-conductivity sensor as recitedin claim 1, wherein the diaphragm has an area in a range of 0.25-2 μm.7. The micromechanical thermal-conductivity sensor as recited in claim1, wherein the diaphragm has a thickness in a range of 0.6-2 μm.
 8. Themicromechanical thermal-conductivity sensor as recited in claim 1,wherein the cavity has a depth in a range of 0.1-2 μm.
 9. Themicromechanical thermal-conductivity sensor as recited in claim 1,wherein the porous cover plate filters a surrounding gas or gas mixture.10. The micromechanical thermal-conductivity sensor as recited in claim1, wherein the heating element and the temperature-dependent electricalresistor are made of one of silver (Ag), gold (Au), nickel (Ni) orplatinum (Pt).
 11. The micromechanical thermal-conductivity sensor asrecited in claim 1, wherein the heating element has a differentcross-section for contacting and in a region of a heating zone.
 12. Themicromechanical thermal-conductivity sensor as recited in claim 1,further comprising: a passivation layer, the at least onetemperature-dependent electrical resistor and the at least one furthertemperature-dependent electrical resistor being protected against aninfluence of chemically aggressive gases or gas mixtures by thepassivation layer.
 13. The micromechanical thermal-conductivity sensoras recited in claim 12, wherein the passivation layer includes a siliconcompound.
 14. The micromechanical thermal-conductivity sensor as recitedin claim 13, wherein the silicon compound is one of silicon oxide (SiO₂)or silicon nitride (Si₃N₄).
 15. A method of using a micromechanicalthermal-conductivity sensor, comprising: providing the micromechanicalthermal conductivity sensor, the sensor including a thermally insulateddiaphragm formed by a recess in a base plate exhibiting poor thermalconductivity; at least one heating element applied on the diaphragm; atleast one temperature-dependent electrical resistor applied on thediaphragm for measuring a temperature of the diaphragm; at least onefurther temperature-dependent electrical resistor applied outside thediaphragm on the base plate for measuring an ambient temperature;wherein the diaphragm is being covered on at least one of its sides by aporous cover plate with a plurality of pores, the at least one porouscover plate which permits gas exchange through diffusion, a cavity beingleft open between the diaphragm and the porous cover plate, the porouscover plate being secured on the base plate, and the porous cover platebeing made of at least one of a porous ceramic material or at leastpartially of porous silicon; and quantitatively analyzing one of a gasand a gas mixture using the sensor.
 16. The method as recited in claim15, wherein the gas mixture contains at least one of hydrogen (H2) andhelium (He).